The invention relates to vacuum brake boosters having a vacuum chamber and a working chamber separated from one another in a pressure-proof manner by a movable wall, having a control valve, which comprises a housing workingly coupled to the movable wall and containing a valve seat, which to achieve a pressure difference at the movable wall is capable of controlling the supply of atmospheric pressure or above-atmospheric pressure to the working chamber in dependence upon the displacement of an input element of the brake booster, as well as having an emergency braking aid comprising a permanent magnet and an armature, which cooperates with the permanent magnet and is spring-biased counter to the actuating direction and in the event of an emergency braking operation is pulled into abutment with the permanent magnet, with the result that the control valve is opened for the supply of atmospheric pressure or above-atmospheric pressure to the working chamber.
Vacuum brake boosters have been known for some time and millions of them are in use for boosting the actuating forces of a vehicle hydraulic brake system and therefore keeping them at a comfortably low level for the driver of a vehicle.
Also known are so-called brake assists. By said term is usually meant a system, which in the event of an emergency braking operation for substantially the same actuating force may provide a driver with an increased braking power. Systems of said type were developed because studies have shown that the majority of vehicle users during an emergency braking operation do not press as hard on the brake pedal as would be necessary to achieve the maximum braking power. The stopping distance of the vehicle is therefore longer than necessary.
Systems of said type already in production employ an electromagnetically actuable brake booster in conjunction with a device capable of determining the actuating speed of the brake pedal. If said device detects an actuating speed above a defined threshold value, it is assumed that an emergency braking situation exists and the brake booster is set to saturation point by the electromagnetic actuating device, i.e. the brake booster supplies its maximum boosting power.
Brake boosters with an electromagnetic actuating facility are however too expensive for motor vehicles in the low to medium price category. There was therefore a demand for solutions, which achieve a brake assist function with a lower outlay.
An, in said respect, improved vacuum brake booster with brake assist function is known from JP 175 373 A. The known vacuum brake booster has a vacuum chamber and a working chamber separated in a pressure-proof manner from one another by a movable wall. A control valve, which has a housing workingly coupled to the movable wall, has an atmosphere valve seat, which is rigidly connected to the housing and which to achieve a pressure difference at the movable wall is capable of controlling the supply of atmospheric pressure to the working chamber in dependence upon the displacement of an input element of the brake booster. For improved braking force assistance during emergency braking operations, an armature cooperating with a permanent magnet is provided in the control valve housing and is coupled in actuating direction rigidly to the actuating element. The armature is spring-biased counter to actuating direction and in the normal position of the control valve is held at a first distance from the permanent magnet. In the course of an approach towards the permanent magnet the armature, when it is less than a previously defined second distance away, which is smaller than the first distance, is pulled by the permanent magnet counter to the spring bias acting on the armature and with simultaneous cancellation of an, in actuating direction, rigid coupling to the actuating element into abutment with the permanent magnet.
The armature substantially takes the form of a hollow cylinder, on the opposite ends of which radially outwardly projecting flanges are disposed. The flange facing the permanent magnet is pulled into abutment with the permanent magnet upon undershooting of the second distance. The flange of the armature remote from the permanent magnet has at its greatest diameter a vacuum valve seat. The permanent magnet is fastened in a mounting, which is connected counter to the actuating direction of the actuating element rigidly to the control valve housing.
In the case of the vacuum brake booster known from JP 175 373 A it was discovered that, in order to achieve a uniform performance of the brake assist, the components configuring the emergency braking aid should have only extremely narrow component tolerances. Otherwise, it is in particular impossible to guarantee the tripping threshold of the brake assist with the required constancy. The close component tolerances of the brake assist resulting from said requirement make mass production difficult and add to the cost of manufacture of the vacuum brake booster.
The object of the invention is to provide a vacuum brake booster of the described type with a mechanical brake assist, which despite generous component tolerances makes it possible to guarantee a uniform performance of the vacuum brake booster.
Said object is achieved according to the invention by a vacuum brake booster having the features indicated in claim 1. The sub-claims relate to advantageous refinements and developments of the invention.
According to the invention it is proposed, for adjusting the distance between the armature and the permanent magnet particularly in the non-actuated state of the vacuum brake booster, to make the axial position of the permanent magnet relative to the control valve housing and/or the distance between the armature and the valve seat adjustable. This has the advantage that all of the components of the vacuum brake booster and, in particular, the components of the brake assist may be manufactured with generous component tolerances. According to the invention, the purposeful adjustment of the distance between armature and permanent magnet is not effected until during or after assembly of the vacuum brake booster.
The adjustment of the distance between the armature and the permanent magnet may be realized in various ways. If, for example, armature and valve seat are of an integral construction, the axial position of the permanent magnet relative to the housing may be adjusted. If, on the other hand, the axial position of the permanent magnet relative to the housing is permanently defined, then armature and valve seat may be provided in the form of separate components, which are connected to one another at a defined maximum distance from one another. Naturally, it is also possible to combine said options for adjusting the distance between the armature and the permanent magnet.
If armature and valve seat are provided in the form of separate components, a defined distance between said two components may be realized in various ways. For example, a distance element may be disposed between armature and valve seat. A set of distance elements of different defined lengths may then be provided and during assembly of the vacuum brake booster, depending on the desired distance between armature and valve seat, the appropriate distance element may be selected and disposed between armature and valve seat. An alternative form of construction provides that deformable standard distance elements are provided, which in accordance with the desired distance between armature and valve seat are reshaped to the required dimension prior to assembly of the vacuum brake booster. The reshaped distance element is then disposed between the armature and the valve seat.
A defined distance between armature and valve seat may also be realized in that armature and valve seat have mutually complementary threads. During assembly of the vacuum brake booster a screw connection is therefore established between the armature and the valve seat, wherein by means of the length of the screw connection the distance between armature and valve seat is adjustable.
A further possibility of connecting armature and valve seat is to provide an interference fit, i.e. a force locking connection between armature and valve seat. The distance between armature and valve seat is then defined by the length of the interference fit connection.
Armature or valve seat or both components may alternatively have an axially extending, deformable extension. The extension or extensions are preferentially disposed between the armature and the valve seat and therefore determine the distance between armature and valve seat. The deformation of an extension may occur as an effect of a force acting on the extension before or during the assembly of armature and valve seat.
Naturally, the previously described options for adjustment of the distance between armature and valve seat may be combined in any desired manner. Thus, for example, by means of distance elements inserted between armature and valve seat the length of the screw connection or of the interference fit connection may be defined.
There are also various options available for adjusting the axial position of the permanent magnet relative to the control valve housing. For example, the axial position is adjustable by means of a distance element, which is disposed between an end face of the permanent magnet facing an input element of the brake booster or between a mounting for the permanent magnet, on the one hand, and an end face of the control valve housing facing the working chamber, on the other hand. The axial position of the permanent magnet relative to the housing may then be adjusted e.g. by means of the length of the distance element. As already described above, a set of distance elements of defined length may be provided or the distance element may be of a deformable design.
The distance element is preferentially of a circular ring-shaped construction and may concentrically surround the permanent magnet or a mounting for the permanent magnet. In said case, the axial position of the permanent magnet relative to the control valve housing is also adjustable by fixing the distance element, on the one hand, and the permanent magnet or the mounting of the permanent magnet, on the other hand, relative to one another by means of an interference fit and purposefully adjusting the length of the interference fit connection.
In the case of the just described options for adjusting the axial position of the permanent magnet relative to the control valve housing, the permanent magnet or the mounting for the permanent magnet is preferentially biased in the direction of the armature by means of an elastic element, e.g. a spring.
A further form of construction for adjusting the axial position of the permanent magnet relative to the control valve housing provides for the connection to the permanent magnet or to a mounting for the permanent magnet of an extension, which extends from the permanent magnet or from the mounting in the direction of the working chamber. By means of said extension the permanent magnet or its mounting may be coupled counter to the actuating direction of an input element of the vacuum brake booster rigidly to the housing. The extension may comprise one or more arms which, to prevent a rotational movement of the permanent magnet, extend through corresponding openings of the control valve housing or of a housing insert connected counter to actuating direction rigidly to the control valve housing.
By means of the extension or extensions of the permanent magnet or of its mounting the axial position of the permanent magnet relative to the control valve housing may be adjusted in various ways. Thus, for example, an extension may have on an end facing the working chamber a radially outwardly extending flange, which engages behind a distance element. The distance element is in turn connected counter to the actuating direction of the input element rigidly to the control valve housing. By selecting a distance element of appropriate length or by reshaping a standard distance element the axial position of the magnet relative to the control valve housing may then be defined.
According to a preferred form of construction, the at least one extension has on an end facing the working chamber a thread, which cooperates with a complementary thread, which is connected counter to the actuating direction of the input element rigidly to the control valve housing, of e.g. a setting ring. To prevent a rotation of the setting ring from being transmitted to the permanent magnet or its mounting, the extension has one or more arms, which extend throughxe2x80x94with regard to a rotational movementxe2x80x94fixed openings of the control valve housing or of a control valve housing insert. The setting ring then cooperates with the regions of the arms of the extension, which extend through said openings. Such a refinement of the vacuum brake booster allows adjustment of the distance between armature and permanent magnet even after assembly of the brake booster, e.g. at preset servicing intervals.
In addition, in the region between the distance element, which cooperates with the flange of the extension, or the setting ring, on the one hand, and the permanent magnet or its mounting, on the other hand, a biased elastic element may be disposed, which presses the distance element and the setting ring, on the one hand, and the permanent magnet or its mounting, on the other hand, in opposite directions.
The vacuum brake booster may comprise a force output element, e.g. in the form of a reaction piston of the master cylinder of a vehicle hydraulic brake system, which element is rotatable relative to the control valve housing and is coupled, with regard to a rotational movement, rigidly to the setting ring. A rotational movement of the force output element is therefore transmitted to the setting ring so that the latter may be conveniently actuated from the outside even after assembly of the brake booster.
Preferably, both the force output element and the setting ring each have at least one axially extending opening, wherein in each case a force transmission element extends through aligned openings of setting ring and force output element. The force transmission element, which may for example take the form of a pin, enables the transmission of a torque from the force output element to the setting ring. To facilitate the introduction of a torque into the force output element, the latter may additionally be provided with a structure in the form of e.g. bumps or indentations, which enable the application of a tool.
According to a further form of construction, the control valve housing at its end facing the working chamber is closed by an insert, which is rotatably disposed inside the control valve housing. Said insert is coupled by a thread to the permanent magnet, which in turn is connected, with regard to a rotational movement about a longitudinal axis of the control valve housing, rigidly to the control valve housing. Because of said, with regard to a rotational movement, rigid coupling of the permanent magnet to the control valve housing, upon rotation of the control valve housing insert relative to the control valve housing the permanent magnet may not co-rotate. Rather, because of the coupling of control valve housing insert and permanent magnet by means of a thread, a rotation of the control valve housing insert effects an axial displacement of the permanent magnet relative to the control valve housing. Since a rotation of the control valve housing insert is still possible even after assembly of the brake booster, said form of construction allows subsequent adjustment of the distance between permanent magnet and armature e.g. in the course of a service inspection.
To facilitate the introduction of a torque into the control valve housing insert, the latter may be provided with an appropriate structure. Said structure may comprise e.g. bumps or indentations, which enable the application of a tool.
The valve seat, which is capable of controlling the supply of at least atmospheric pressure to the working chamber, is preferentially coupled at least in actuating direction both to the input element of the brake booster and to the armature. Such a refinement of the valve seat allows an uncomplicated supply of at least atmospheric pressure to the working chamber in dependence upon the displacement of the input element.